Not tokens, the Memo Key is currently "un-used" but is for signing "memos" or messages. We plan to leverage this for an authenticated and secure chat protocol built on rpc and the steemit blockchain ;) this is exactly what the white papers outline is the reason for this key to begin with yet steemit.com still says and I quote:

Memo key - Currently the memo key is not used.
https://steemit.com/faq.html#What_are_my_different_keys_for

;) this is going to change this up big time!